Breast cancers often have increased mitogen-activated protein kinase (MARK) activity, and this pathway influences breast cancer cell growth in part by targeting steroid hormone receptors. Overexpression of signaling molecules that input to MAPKs confer tamoxifen resistance. Estrogen receptor-alpha (ER) and progesterone receptors (PR) are required for normal breast development and essential for the development and progression of the majority of breast cancers. These receptors function independently, but also clearly interact to regulate hormone responsiveness. For example, PR can exhibit autoinhibition via regulation of the PR N-terminus, and transrepression by suppression of the transcriptional activities of ER. Post- translational modification of the PR N-terminus by covalent attachment of SUMO-1 (sumoylation) is required for both PR autoinhibition and transrepression of ER. The PR N-terminus is also heavily phosphorylated by multiple protein kinases. Phosphorylated PR-B are under-sumoylated, thus predicting the derepression of PR and ER activities and increased responsiveness to estrogens. We hypothesize that hyper- phosphorylation of the PR N-terminus leads to loss of PR sumoylation and altered regulation of PR and ER function in advanced breast cancers that express functional receptors, yet are resistant to steroid hormone- based therapies. Thus, growth factors may usurp steroid hormone control of breast cancer cell growth by direct phosphorylation of PR, in turn altering PR sumoylation and critical PR/ER interactions, thereby influencing the tumor response to anti-estrogens. We will test our novel hypothesis using both in vitro and in vivo approaches. Questions for study include; 1) Do growth factors and/or steroid hormones differentially regulate PR sumoylation in a phosphorylation-dependent manner? 2) Do growth factors and/or steroid hormones modulate ER and PR activities in a SUMO-dependent manner? and 3) Can sumoylation of PR alter estrogen-dependent breast cancer cell growth and/or reverse the sensitivity of tamoxifen-resistant tumors in vivo? Understanding the mechanisms of growth factor control of steroid hormone function will allow the exploitation of novel pathways for breast cancer treatment. [unreadable] [unreadable] [unreadable]